Recently, blue region (which means to be of a wavelength range of ultra-violet to yellow colors) light emitting diodes (hereinafter abbreviated as LEDs) for use as a light source used in a full-color display and signal lamp or blue region semiconductor lasers (hereinafter abbreviated as LDs) for a light source used in a next-generation high-definition DVD capable of continuous oscillation at room temperature, were developed by growing GaN-based compound semiconductor layers on the C-plane of a sapphire substrate, and thereby drawing attention from the industries.
As shown in FIG. 14 illustrating a perspective explanation view of an LD chip, this structure comprises a sapphire substrate 21 and group III nitride compound semiconductor layers sequentially grown thereon by Metal Organic Chemical Vapor Deposition (hereinafter abbreviated as MOCVD), in such a configuration that a GaN buffer layer 22, an n-type GaN layer 23, an n-type clad layer 24 made of Al0.12Ga0.88N, an n-type light guide layer 25 made of GaN, an active layer 26 having a multi-quantum well structure made of InGaN-based compound, a p-type light guide layer 27 made of p-type GaN, a first p-type clad layer 28a made of p-type Al0.2Ga0.8N, a second p-type clad layer 28b made of Al0.12Ga0.88 N, and a contact layer 29 made of p-type GaN are stacked sequentially and parts of these stacked semiconductor layers are dry-etched, as shown in FIG. 14, to expose the n-type GaN layer 23 on which an n-side electrode 31 is formed with a p-side electrode 30 being formed on the contact layer 29.
The ZnO based compound semiconductor, on the other hand, has been studied in a variety of aspects because it has wide-gap energy semiconductor so that Cd can be mixed in crystallinity to narrow the band gap energy and also emit a blue region light similarly and also because it can be used in a SAW device, a pyro-electric device, a piezoelectric device, etc. This ZnO based compound semiconductor is also a hexagonal crystal like GaN based compound and sapphire, and so has an approximate value of the lattice constant, so that (0001) sapphire having a C-plane as its main plane which is used generally as a substrate for growing epitaxially GaN based compound semiconductor layers in the industries is expected to be used as a substrate for growing ZnO based compound semiconductor layers.
The growth of a ZnO based compound semiconductor on the (0001) sapphire substrate is described in, for example, “Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films”, Applied Physics Letters, Vo. 72, No. 25, issued on Jun. 22, 1998, pp. 3270-3272.
As mentioned above, since a substrate employed in the prior art blue region semiconductor light emitting device is made of (0001) sapphire having the C-plane as its main plane, it has no conductivity and cannot give therein a vertical-type device (which means to have a construction in which electrodes are provided on both right and back sides of the chip) that has electrodes on both its top and bottom faces of a stack. These electrodes, therefore, must be provided on the upper surface of the semiconductor layer lamination and the surface of its underlying semiconductor layer exposed by etching part of the upper layers, thus giving birth to a problem of complicating manufacturing processes such as etching process and chip die bonding process. Moreover, since the sapphire substrate is very hard, it cannot be cloven easily and is difficult to form a flat end face necessary as the mirror surface of an optical resonator for a semiconductor laser, problematically. That is, although the sapphire substrate is capable of obtaining a well-conditioned mono-crystal semiconductor layer, it has an inevitable difficulty in processibility and formation of electrodes during the manufacturing process.
Further, sapphire has a c-axial length CS of 1.2991 nm and an a-axial length as of 0.4754 nm, while ZnO has a c-axial length cz of 0.5213 nm and an a-axial length aZ of 0.325 nm, so that the lattice mismatching rate ε becomes a very large value of ε=(az−as)/as=−31.6%. In this case, as shown in FIG. 15, the ZnO crystal may sometimes grow as rotated by 30 degrees, even in which case, the crystal mismatching degree ε has a very large value of ε=(⅔1/2·aZ−zS)/aS=−21.1%. This brings about complicated actions of such various parameters as a substrate temperature at the time of crystal growth, amounts of Zn and O elements supplied, a substrate surface treatment method, and an inclination angle, thus giving birth to a problem of poor reproducibility of the flatness of a crystal-growing surface.
Also, since sapphire and ZnO mismatch in lattice constant with each other, the ZnO crystal may sometimes grow as rotated by 30 degrees as mentioned above, so that there are mixed a crystal not rotated and a crystal rotated by 30 degrees, thus giving birth to a problem of even poorer reproducibility of the flatness of the crystal growing surface.
In view of the above, it is a first object of the present invention to provide a semiconductor light emitting device such as an LED or LD made of a ZnO based compound semiconductor, that is of a vertical type capable of providing electrodes from both right and back sides of a chip thereof, that has excellent crystallinity of a semiconductor layer thereof and a good light emitting efficiency, and that does not use sapphire as a material of a substrate thereof to thereby provide a convenient construction in both manufacture and use.
It is a second object of the present invention to provide a semiconductor light emitting device manufacturing method involving surface treatment of a silicon substrate fitted especially to a purpose of growing a ZnO based compound semiconductor on the silicon substrate with good crystallinity.
It is a third object of the present invention to provide a device using a ZnO based compound such as a semiconductor light emitting device with improved device properties which gives a ZnO based compound crystal layer with good crystallinity even on a sapphire substrate.
It is a fourth object of the present invention to provide such a method for growing a ZnO based compound layer that is capable of giving a ZnO based compound crystal layer with excellent crystallinity even on a sapphire substrate.
It is a fifth object of the present invention to provide a semiconductor light emitting device such as an LED or LD having excellent light emitting properties which employs a ZnO based compound semiconductor while using a sapphire substrate.